The present invention relates to the calendering of paper webs to impart gloss to the paper, and more particularly, to a method and apparatus for minimizing microgloss variations and microgalvanizing in high gloss papers.
Gloss is imparted to paper webs, e.g., to coated printing papers, by the highly developed and well known art of calendering. Various types of calenders are employed in the paper industry, including hard nip calenders, soft nip calenders, supercalenders, and most recently, soft heated extended nip calenders of both belt and shoe types.
Supercalendering has generally been deemed to be the most effective method of obtaining extremely high surface gloss. Supercalendering has, however, been identified as contributing to variations in surface gloss which can result in a nonuniform, mottled appearance or orange peel effect that adversely affects the print quality of the paper. The resultant undesirable appearance is referred to as microgalvanizing and/or submillimeter and microgloss variations.
It is the object of the present invention to control microgloss variations and minimize microgalvanizing in high gloss paper, especially paper that has been supercalendered.
In accordance with the invention, it has been determined that microgalvanizing can be reduced and minimized by passing fully super-calendered webs through one or more calendering nips of a heated soft extended nip calender.
In particular, it has been determined pursuant to the invention that even a single pass of supercalendered paper through a very soft extended nip after the paper gloss has been fully developed will significantly reduce microgalvanizing while retaining the supercalendered paper gloss.
These and other features and advantages of the invention will become apparent to those reasonably skilled in the art from the following detailed description, as considered in connection with the accompanying drawings.